This invention relates generally to processes that produce seals or welds between plastic sheets, as for example polyvinyl chloride plastic sheets, important to the art of plastic goods manufacture. More particularly, it concerns the attainment of more perfect (i.e. high strength) welds between such sheets.
A number of these prior processes employ heat to soften, semi-liquefy, or liquefy regions of sheet which are held together, under pressure, until cool to produce welds. The heat employed may be conducted into sheet regions from heated solid dies, convected and conducted into sheet regions from hot air or gas, produced within sheet regions by molecular vibrations induced by rapidly varying electric fields (radio frequency heat sealing) or rapidly varying elastic compressions (ultrasonic heat sealing), or some combination of these.
The methods presently employed in welding together plastic sheets (with the exception of methods using filler rods analogous to welding rod used with metals) produce two types of welds.
The "Butt Weld" between plastic sheets (a misnomer relative to metal welding parlance) is produced by sealing two layers of plastic sheets together at overlaying edges. In use, the two sheets are ordinarily unfolded so that the "Butt Weld" forms a more or less distorted and puckered seal between two sheet edges. When stressed, the "Butt Weld" usually causes the virgin sheet to break immediately adjacent to the "Butt Weld" at a stress of 60 to 65% of its breaking strength at a distance from the "Butt Weld."
The "Lap Weld" is produced by sealing two plastic sheets together at overlapping edges.
When stressed the "Lap Weld" usually causes the virgin sheet to break immediately adjacent to the "Lap Weld" at a stress of 80 to 85% of its breaking strength at a distance from the "Lap Weld."
The lower plastic sheet breaking strengths adjacent to "Butt Welds" and "Lap Welds" are well explained by the theory of elasticity which is confirmed by experimental methods. In both cases the redistribution of stresses adjacent to and across the welds results in stress concentrations adjacent to the welds in excess of the essentially uniform stresses across sheets at substantial distances from welds (St. Venant's Principle).